1. Field of the Invention
The present invention relates to an axle driving apparatus comprising a pair of axles differentially connected to each other, a propelling transmission mechanism for rotating both the axles together at various speeds in the same direction alternatively selected from two opposite directions for forward and backward traveling of the vehicle, and a steering transmission mechanism for transmitting oppositely directive forces to the respective axles so as to generate a difference of rotational speed between the axles, and wherein the axle driving apparatus is provided for a vehicle (e.g., a mower tractor, installed with a belt type stepless transmission).
Also, the present invention relates to a steering mechanism of a caster which is available for a working vehicle such as a mower tractor requiring a small turn radius, wherein the working vehicle is provided with an axle driving apparatus comprising a propelling transmission mechanism that transmits propelling force to both mutually differentially connected axles and a steering transmission mechanism that transmits oppositely directive forces to the respective axles.
2. Related Art 
Conventionally, as disclosed in Japan Patent Laid Open Gazette Nos. Sho 62-28,554 and Hei 1-234,644, and U.S. Pat. Nos. 6,141,947 and 4,771,856, and others, there is a well-known belt type stepless transmission having a double pulley, namely, a variator. Additionally, there is a well-known vehicle such as a mower tractor having the belt type stepless transmission, wherein first and second belts are looped over respective pulley portions of the double pulley so that the tension of the first belt is changed so as to change the rate of driving speed of the subsequent second belt to that of the first belt.
Especially, Japan Patent Laid Open Gazette No. Hei 1-234,644 discloses a vehicle which utilizes a speed difference between the first and second belts looped over the variator so as to change its traveling direction and speed.
The above-mentioned belt type stepless transmission using the variator is advantageouss to working vehicles such as a mower tractor requiring stepless variation of traveling speed because it is more economical than a hydrostatic stepless transmission (an HST).
Also, conventionally, there is a well-known vehicle having a steering transmission which transmits power of a prime mover in two ways so as to make a difference of rotational speed between left and right drive wheels, that is, accelerate one drive wheel and decelerate the other according to the operation of a steering device such as a steering wheel. An HST may serve as the steering transmission.
However, none of the above-mentioned documents discloses such a steering transmission. More specifically, there is no conventional vehicle having a belt type stepless transmission as mentioned above which transmits its output to such a steering transmission. Such a driving combination of the belt type stepless transmission and the steering transmission, if it is invented, will be advantageous to provide an economical working vehicle.
Furthermore, conventionally, there is a well-known small working vehicle like a mower tractor which has at least one caster for enabling a very small turn radius. However, a conventional caster provided on such a working vehicle is not steered by a steering device like a steering wheel.
A caster which is not steerable has a disadvantage in its weakness against a side force. More specifically, if a vehicle, e.g., a mower tractor with such a caster descending a slope slantwise, the caster is oriented more downward than the driving direction of the vehicle because the weight of the vehicle is applied as a side force onto the caster. Therefore, the vehicle can descend slantwise comparatively smoothly. Then, if the vehicle is going to ascend the slope slantwise, the downwardly oriented caster is hard to turn upward because of the side force. Therefore, the vehicle cannot ascend slantwise smoothly.
Thus, it is required that a steering operational force of the steering device be forcibly applied onto the caster so as to steer the caster without disturbing the steering operation while the wheels of the casters are allowed to rotate freely according to the force applied from the ground regardless of the steering condition.
An object of the present invention is to provide an axle driving apparatus for an economical vehicle which turn has a small turn radius and whose traveling speed varies steplessly.
To achieve the object, an axle driving apparatus according to the present invention is suitable to a vehicle having a prime mover and a belt type stepless transmission. The axle driving apparatus comprises a pair of axles connected to respective drive wheels, a propelling transmission mechanism and a steering transmission mechanism. The propelling transmission mechanism is drivingly connected to said prime mover through said belt type stepless transmission so as to transmit speed-variable output rotation of said belt type stepless transmission to both said axles, thereby rotating said axles together at various speeds in a common direction selected from two opposite directions for forward and backward traveling of said vehicle. The steering transmission mechanism is drivingly connected to said prime mover so as to transmit two oppositely directive output rotations to said respective axles, thereby differentially rotating said axles at various differences of rotational speed between said axles.
An accelerator such as a pedal is operatively connected to said belt type transmission so as to change the output rotational speed of said propelling transmission mechanism. A steering operation device such as a steering wheel is operatively connected to said steering transmission so as to change the output rotational speed of said steering transmission.
The steering transmission mechanism may comprise a hydrostatic transmission.
The pair of axles may be differentially connected to each other through a first differential mechanism. In this case, said first differential mechanism transmits the output rotation of said propelling transmission mechanism to both said axles, and allows said axle to be differentially rotated by the oppositely directive output rotations of said steering transmission mechanism transmitted to said respective axles.
Furthermore, a second differential mechanism driven by said steering transmission mechanism may be provided so as to generate two oppositely rotational forces applied onto said respective axles.
A housing of the axle driving apparatus may incorporate the axles, the propelling transmission mechanism and the steering transmission mechanism together. Furthermore, the first and second differential mechanisms may be disposed together in the housing.
The housing may comprise a first chamber in which the axles, the propelling transmission mechanism and the first and second differential mechanisms are disposed, and a second chamber in which the steering transmission mechanism is disposed.
Alternatively, the axle driving apparatus may comprise at least first and second housings. The first housing may incorporate the axles and the propelling transmission mechanism, and further, the first differential mechanism. The second housing may incorporate the steering transmission mechanism, and further, the second transmission mechanism. In this case, a transmission mechanism may be interposed between the first and second housings so as to transmit the two oppositely directive output rotations of the second differential mechanism to the respective axles.
The belt type stepless transmission may be constructed as follows.
A pair of first and second belts are looped over a variator pulley such as a double pulley. The first belt receives the power of the prime mover prior to the second belt. The first belt may be driven at a constant speed determined according to the output rotational speed of the prime mover. The first belt is looped over a first idler pulley for tension variation thereof. The accelerator is operatively connected to the first idler pulley. The second belt is looped over a second idler pulley for tension-variation thereof. The variator changes a driving speed ratio of the second belt to the first belt according to the tension-variation of the first belt by operation of the accelerator.
The axle driving apparatus is basically provided with an input pulley over which the second belt is looped so as to transmit the speed-variable output of the belt type stepless transmission to the propelling transmission mechanism.
For inputting of the steering transmission mechanism, the first belt may be looped over an input pulley of the steering transmission mechanism. If the first belt is driven at the constant speed determined according to the output rotational speed of the prime mover, the input pulley of the steering transmission mechanism is rotated at a constant speed. Furthermore, if the steering transmission mechanism comprises a hydrostatic transmission, a hydraulic pump thereof can be driven at a constant speed.
Alternatively, the input pulley of the propelling transmission mechanism over which the second belt is looped may also be used as an input pulley of the steering transmission mechanism so as to allow the input rotational speed of the steering transmission to vary in proportion to the variation of driving speed of the second belt according to the operation of the accelerator.
Alternatively, a common input device shared between the belt type stepless transmission and the steering transmission mechanism may receive power of the prime mover. If the steering transmission mechanism comprises a hydrostatic transmission, it may be so constructed that a pump shaft of the hydrostatic transmission may be drivingly connected to the common input device. A pulley over which the first belt is looped is fixed onto the pump shaft so as to transmit the rotational force of the pump shaft to the first belt of the belt type stepless transmission.
For switching the rotational direction of the drive wheels between forward and backward, a switching device whose output rotational direction is selected between two opposite directions may be provided.
The switching device may be provided in the propelling transmission mechanism between the belt type stepless transmission and the pair of axles. If the first differential mechanism is provided, the switching device may be interposed between the belt type stepless transmission and the first differential.
If the common input device receiving power of the prime mover is provided, power of the common input device may be transmitted through the switching device to both the steering transmission mechanism and the belt type stepless transmission. When a hydrostatic transmission serves as the steering transmission and the pulley over which the first belt is looped is fixed onto the pump shaft of the hydrostatic transmission, the switching device is interposed between the common input device and the pump shaft. Due to this construction, the operational direction of the steering operation device, e.g., the turning direction of a steering wheel, can coincide with the target side of the vehicle to be turned whether the vehicle travels forward or backward.
The propelling transmission mechanism may comprise a third differential mechanism differentially mutually connecting two pulleys over which the first and second pulleys are looped respectively. If the above-mentioned first differential mechanism is provided, the output rotational force of the third differential mechanism is transmitted to the first differential mechanism so as to rotate the pair of axles together in a common direction at various speeds. Since the output rotational direction of the third differential mechanism can be switched according to the difference of rotational speed between the two pulleys, the traveling direction of a vehicle can be switched between forward and backward only by operation of the accelerator.
The third differential mechanism may be of either a planetary gear type or a bevel gear type. If the third differential mechanism is of a planetary gear type, the third differential may comprise a sun gear, a planet gear and a ring gear. The sun gear is fixed to the pulley over which the first belt is looped. The ring gear includes an outer gear and an inner gear. The planet gear engages between the sun gear and the inner gear of the ring gear. A variable-speed gear is fixed to the pulley over which the second belt is looped and engages with the outer gear of the ring gear. The revolution of the planet gear around the sun gear is transmitted to the pair of axles so as to rotate the pair of axles together at various speeds in a common direction that is selected in either the forward or backward traveling direction.
If the first and third differential mechanisms drivingly connected to each other as mentioned above are provided, the output force of the steering transmission mechanism may be applied onto one of the axles so as to rotate the pair of axles in opposite directions and change the relative speed between the axles instead of the second differential mechanism. In this case, a clutch may be interposed between the output portion of the steering transmission mechanism and the one axle so as to secure a dead zone of the steering operation device. If the steering transmission mechanism comprises a hydrostatic transmission, a bypass replacing or in addition to the clutch may be interposed between a pair of oil passages between a hydraulic pump and a hydraulic motor.
A second object of the present invention is to provide a small turn radius turnable vehicle with a caster improved so that the caster can be adequately directed so as to overcome a sideward resistant force from the ground, comprising a prime mover, a pair of left and right drive wheels, an axle driving apparatus, and a steering operation device, wherein the axle driving apparatus includes a pair of axles driven by the prime mover and drivingly connected to the respective drive wheels and a steering transmission for rotating the pair of axles in opposite directions and changing a relative speed between the pair of axles, and wherein the steering operation device is operatively connected to the steering transmission so as to change the output rotational direction and speed of the steering transmission.
To achieve the second object, the caster is rotated laterally with respect to a vehicle by operating the steering operation device. That is, the torque caused by operation of the steering operation device is transmitted to the caster so as to overcome the sideward force from the ground.
A clutch, which is switched between an engaging condition and a disengaging condition, may be interposed between the caster and the steering operation device. The clutch set in the engaging condition allows a torque generated by the steering operation device to be transmitted to the caster. The clutch set in the disengaging condition prevents the torque generated by the steering operation device from being transmitted to the caster.
The clutch may be switched by electromagnetic control.
Furthermore, a torque diode, a bi-directional clutch or so on prevents a side force applied onto the caster from the ground for swinging the caster laterally about the vehicle from being transmitted to the steering operation device, thereby preventing operation of the steering operation device from being disturbed by the free lateral movement of the caster.
Other and further objects, features and advantages of the invention will appear more fully from the following description.